Electron lens for reduction of spherical aberration



A. P. WILSKA Filed Nov. l5, 1961 Aug. 6, 1963 ELECTRON LENS FOR REDUCTION OF' SPHERICAL ABERRATION Wifi/f7@ E W.

United States Patent O 3,160,260 ELECTRN LENS FR REDUCTION GF SPERICAL ABERRATIN Alvar P. Wiiska, Tucson, Ariz., assignor to Philips Elec tronies and Pharmaceutical industries Corp., New York,

N.Y., a corporation of Maryland Filed Nov. 15, 1961, Ser. No. 152,531 Claims. (Cl. 250-495) This invention relates to an electron lens system for reducing the spherical aberration that is present in all magnetic electron lenses.

In accordance with the present invention an electrostatic lens is placed in the lens region of a magnetic lens in order to correct the spherical aberration of an annular region of the electron beam. The electrostatic lens comprises a central electrode, which is located on the electron beam axis within the magnetic iield of the magnetic lens, and an annular electrode surrounding the central electrode and operated so as to be electrically negative with respect to it. The central electrode is cylindrical and serves as an elongated central stop and the electric field strength between the central electrode and the annular electrode decreases logarithmically, as measured along a radius perpendicular to the beam axis. This radially directed electric field causes greater convergence in 'the inner parts than in the outer parts of the hollow electron beam that passes through the gap between the central electrode and the annular electrode which is opposite to the effect of spherical aberration produced by the magnetic lens. In order properly to define the iield, an additional electrode surrounds the annular electrode except in the gap between the latter and the central electrode. The additional electrode has annular portions spaced from each end of the annular electrode and the inner edges of these annular portions form annular gaps with the outer edges of the ends `of the central electrode to deiine the portion of the beam that will be acted upon by the electric iield.

The invention will be described in greater detail in connection with the drawings, in which:

`FIG. l is a simpliiied representation of an electron beam device having two electron 'lenses with spherical aberration-reducing electron systems in .accordance with the present invention; and

FIG. 2 shows a more detailed cross-sectional View of the electrode structure for use in a lens of the type shown in FIG. 1.

In FIG. 1 an electron beam `device 11 enclosed within a chamber bounded by a wall 12 includes an electron source, or cathode, 13 together wit-h a control electrode, or grid, 14. An accelerating anode 16 is connected to a suitable source of potential to draw electrons `from the cathode 13 along a direction generally parallel to an axis 17. If necessary, additional electrodes or magnetic fields may be provided, as is well known in the electron beam art, to form the electrons into a beam 18.

The electron beam 18 travels into the lens region of a magnetic iield defined by pole pieces 19 and 21 which are at opposite magnetic polarities in order to provide an axial magnetic field in the axial space between the pole pieces. For the sake of simplicity, it may be assumed that the pole pieces 19 and 21 are parts of a structure which is permanently magnetized with north and south poles as indicated by the letters N and S, although it will be understood that the magnetic polarization may be induced by current flowing in a coil of wire properly situated with respect to the pole pieces 19 and 21. Both of the pole pieces 19 and 21 have central apertures deiined by walls 22 and 23, respectively, through which the electron beam 18 passes, and the magnetic field between Patented Aug. 6, 1963 the pole pieces 19 and 211 is substantially rotationally symmetric with respect to the electron beam axis 17.

Located within the magnetic field, that is, within the gap between the pole pieces 19 and 21, is an electrode system comprising an elongated, cylindrical, central electrode Z4 that operates as a central stop to prevent the central portion of the electron beam 18 from continuing along the axis 17. Surrounding the central electrode 2.4 and concentric therewith and spaced therefrom is an annular electrode 26 which, in the embodiment shown, is shorter than the central electrode in a direction measured along the axis 17.

A difference of potential is maintained between the central electrode 24 and the annular electrode 26 and this difference of potential is polarized so as to make the annular electrode negative with respect to the central electrode 24. To give some idea of the magnitude of the potential between these two electrodes, it has been found that a voltage of about 300 bolts is suiiicient to provide satisfactory spherical aberration correction of an electron beam in which the electrons have been accelerated by a voltage eld created by making the anode 16 about 15,000 volts positive with respect to the `cathode `13. In this environment, and with the central electrode operating at substantially the same voltage as the anode 16, a suitable electrostatic field is established between the central i electrode 24 and the annular electrode 26 to correct for spherical aberration of the electron beam or at least of that annular portion of the beam which is able to pass through the gap between the central electrode and the annular electrode.

The effect of the field between the two electrodes is such as to deect more strongly toward the axis 17 those electrons which come closer to the annular electrode 26 than those electrons that pass through the gap in regions closer to the central electrode 24. By appropriate adjustments of both the magnetic and the electric components of the field within the gap between the pole pieces 19 and 211 substantially complete compensation of spherical aberration can be achieved for a narrow annular Zone of electrons.

In order to define more precisely the dimensions of the annular zone of electrons to be atfected by the eld between electrodes 2.4 and 26, an additional electrode Z7 is placed around the electrode 26. As may be seen, the electrode 27 consists of two annular discs 28 and 29 which are spaced from opposite ends of the electrode 26 and the perimeters of which are joined by a short, hollow tube 31 so that the annular electrode Z6 is surrounded on three sides by the additional electrode 2.7.

The annular zone of electrons that is able to pass through the electrode system impinges upon an object `31.?, which may be, for example, a specimen in an electron microscope. Thereafter the annular electron beam 18, having been modified by the specimen 32, is magniiied by a second electron lens in the region between the magnetic pole pieces 33 and v34. This lens system is, in principle, the same as the lens system which was just described, and in addition to the pole pieces 33` and 34 and the rotationally symmetrical axial magnetic field therebetween 4there is an electrode system comprising a central cylindrical electrode 36 surrounded by an annular electrode 37 which, in turn, is surrounded on three `sides by an additional electrode 3S comprising annular discs 39 and 41 joined by a short tube 42. The additional electrode -38 and the central electrode 36 operate at the same electrical potential while the annular electrode 37 `operates at a potential which is negative ywith respect thereto.

The magnified electron image of the specimen 32 may be further magniiied by additional electron lenses, if desired, and may be rendered visible by being directed against a fluorescent screen 43 at the opposite end of the device .11 from the origin of the electron beam 18.

FIG. 2 shows the electrode system of FIG. l in greater detail. The central electrode -124 is in the form of a solid metal post having an outwardly extending flange 44 at its upper end. The upper surface of the flange 44 is at While the lower surface of the ange has a frusto conical shape. The edge of the flange is therefore relatively sharp and electrons which strike the flange either land on top of it or at the very edge, but they cannot strike the tapered surface. Therefore, if bits of dust become attached to the tapered surface, they do not become charged by the electrons of the beam. A similar frustoconical flange 46 is located at the other end of the electrode 124 and is provided with a tapered under-surface similar to that of flange 44 and for the same purpose.

The annular electrode 126 is in the form of a metal ring which surrounds the central area of the central electrode 17A and is spaced at some distance from the central electrode and is substantially concentric therewith. The annular electrode is supported by insulating means, such as ceramic posts 47 and 48. Post 48 is formed with a channel through it to permit an electrode lead 49 to be connected to the annular electrode 126 in order to operate this electrode at the desired voltage.

The insulating posts 47 and 48 are, in turn, supported by a hollow cylinder `131, which is part of an additional electrode 127. The additional electrode 12.7 has two annular disc portions 12S and 129, the latter being formed integrally with the cylindrical portion 131 Iwhile the disc portion 128 is bolted to the other end of the cylindrical portion 131 by a plurality of machine screws 51. Both of the disc portions 12S and 129 have central apertures which ,are concentric -with the axis 17 that passes through the center of the electrodes 124 and i126. Furthermore, both of the annular disc portions 12S and -129 are formed so that the edges of their respective apertures have tapered surfaces 52 and 53, respectively. These tapered surfaces are provided for the :same purpose as the tapered surfaces of the anges 44 and `46 and in addition the sharp edge of the disc portion 128 is substantially coplanar with the sharp edge of the flange 44 while the sharp edge of the disc portion 129 is substantially coplanar Iwith the sharp edge of the lower flange 46. These edges dene the annular zone of the electron beam 18 that can pass through the electrode system.

The electrode system is supported from the wall 1-2 of the electron beam device on a flat plate 54 which has a central opening 56 into which an axial extension 57 of the central electrode 124 is inserted. A plurality of apertures 58 in the plate 54 permits the electrons of the annular zone of the electron beam 18 to pass through and to continue toward the utilization area of the device. These apertures 58 may be of circular cross-section, if desired, or they may be in the form of radial tins, provided that they are suiiiciently strong to support the central electron rigidly. The upper surface of the plate S4 has an annular groove 59 directly over the apertures 58 for the purpose of removing the upper surface of the ns or of the plate as Afar as possible, consonant with requirements of rigidity, from the region of the electrode system so that the electric iield with the electrode system will be substantially unalected by the plate 54. Since the electrodes 124 and 127 are normally operated at the same electrical potential, the metal plate 54 serves as a suitable means for connecting the proper potential to the central electrode 124.y

While this invention Vhas been described in specific terms, it will be realized by those skilled in the art that the embodiments shown do not constitute limitations on the invention but on the contrary the true scope of the invention is determined by the following claims.

What is claimed is:

l. An electron lens comprising means for producing a rotationally symmetric magnetic iield along an electron beam axis; a central electrode located on said beam axis Within said magnetic field; and a second electrode annularly spaced from said central electrode and substantially coaxial therewith, said second electrode being operable at a negative voltage with respect to said central electrode to cause electrons passing through the annular space between said electrodes to be converged toward said axis more strongly the closer said electrons are to said central electrode.

2. An electron lens comprising means for producing a rotationally symmetric magnetic eld along an electron beam axis; an elongated, cylindrical, central electrode located on said beam axis Within said lmagnetic: field; and a second electrode annularly spaced from a central region of said central electrode and substantially coaxial there with, said second electrode being operable at a negative voltage with respectV to said central electrode to cause electrons passing through the annular space between said electrodes to be converged toward said axis more strongly the closer said electrons are to said central electrode.

3. In an electron beam device in which the `electron beam travels lalong a beam axis, an electron lens on said axis comprising: means for forming a substantially rotationally symmetric magnetic eld on said axis; a cylindrical central electrode within said iield and coaxial with said beam axis; a first annular electrode concentrically spaced from said central electrode and coaxial therewith and negatively electrically polarized with respect to said central electrode; and a second annular electrode comprising a first annular portion axially spaced from said first annular electrode in one direction along said beam axis and a second annular portion axially spaced from said rst annular electrode in the opposite direction along said beam axis, said second -annular electrode having substantially the same electrical potential as said central electrode and being coaxial therewith.

4. The electrode system of claim 3 in which the central apertures of Said first and second annular portions are smaller than the central opening in said first annular electrode whereby said first annular electrode is shielded on both axial ends by -said first and second annular portions.

5. ln an electron beam device in which the electron beam travels along a beam axis, an electron lens on said axis comprising: a rotationally symmetric magnetic lield substantially coaxial with said beam axis; and an electrode structure located Within said magnetic iield and comprising a central `electrode on said beam axis, a first annular electrode surrounding said central electrode and electrically insulated therefrom, said first annular electrode being electrically negative with respect to said central electrode, a second annular electrode substantially coaxial with said central electnode and axially displaced from said rst annular electrode in one direction along said beam axis, and a third annular electrode substantially coaxial with said central electrode and axially displaced vfrom said first annular electrode in the opposite direction along said beam axis, lsaid second and third annular electrodes being at substantially the same electrical potential as said central electrode.

6. In an electron beam device in which the electron beam travels along a beam axis, an electron lens on said axis comprising: a rotationally symmetric magnetic field substantially coaxial with said beam axis; and an electrode structure located within said magnetic eld and comprising an electron-impermeable central electrode on said beam axis, a irst annular electrode surrounding the central region of said central electrode and spaced and electrically insulated therefrom, said first annular electrode being electrically negative with respect to said central electrode, a second annular electrode coaxial with said central electrode and axially displaced from said rst annular electrode in one direction along said beam axis; and a third :annular electrode coaxial with said central electrode and axially displaced from said tirst an- 5 nular electrode in the `opposite direction along said beam axis, said second and third annular electrodes being at substantially the Same electrical potential as said central electrode.

7. An electron lens comprising means for producing a rotationally symmetric magnetic field along an electron beam axis; a cylindrical central electrode located `on said vbeam axis within said magnetic iield; a second electrode annularly spaced from a central region of said -central electrode and substantially coaxial therewith, said second electrode being operable at a negative voltage with respect to said central electrode to cause electrons passing through the annular space between said electrodes to be converged toward said axis more strongly the closer said electrons are to said central electrode; and a third electrode electrically connected to said central electrode and comprising a tubular portion surrounding said second electrode and insulated therefrom and a pair `of annular discs extending inwardly toward said central electrode to form a pair of annular electron beam gaps therewith.

8. An electron lens comprising means for producing a rotationally symmetric magnetic eld along an electron beam axis; an elongated, cylindrical, central electrode located on said beam axis within said magnetic field; a pair 0f laterally extending :anges near each end of said central electrode; a second electrode annularly spaced from a central region of said central electrode and substantially coaxial therewith, said second electrode being operable at a negative voltage With respect to said central electrode to cause electrons passing through the annular space between said electrodes to be converged toward said axis more strongly the closer said electrons are to said central electrode; and a third electrode cornprising a tubular portion substantially coaxial with said second electrode and annularly spaced therefrom and a pair of annular discs extending inwardly from said tubular portion and substantially coplanar -with said flanges to form annular electron beam gaps therewith.

9. In an electron beam device in which the electron beam travels along a beam axis, an electron lens ron said axis comprising: a rotationally symmetric magnetic field substantially coaxial with said beam axis; -a cylindrical electron-impermeable central electrode having an axis substantially -coincident with said beam axis and being located within said magnetic eld; a rst hollow cylindrical electrode surrounding a central region of said central electrode and concentrically spaced therefrom and coaxial therewith and electrically negative with respect to said central electrode; and a third electrode comprising a rst annular disc portion substantially coplanar with one end of said central electrode yand axially spaced from said hollow cylindrical electrode in one direction along said beam axis, a second annular disc portion substantially coplanar with the yother end of said central electrode and axially spaced from said hollow cylindrical electrode in the opposite direction along said beam axis, and a portion joining said rst and second 4disc portions outside of said hollow cylindrical electrode, said third electrode having substantially the same electrical potential as said central electrode with Athe centers of the apertures of both of said disc portions being located on said beam axis.

l0. rThe electron lens of claim 9 lin which said central electrode has la radial llange at each end thereof substantially coplanar with said lirst anfd second disc portions, respectively.

ll. The electron lens of claim l0A in which each of said radial flanges has a ilat surface facing in the direction of origin `of the electron beam and ya conical surface facing in the opposite direction and tapered inwardly, and each of said annular disc portions has a centnal aperture defined by an edge which is substantially flat in the direction facing the origin of `the electron beam and an outwardly tapered surface extending longitudinally along the thickness of said annular disc portions.

l2. in an electron beam `device in which the electrons travel along a beam axis, an electron lens on said axis comprising: a rotationally symmetric magnetic field substantially coaxial with said beam axis; a cylindrical electron-impermeable central electrode having an axis substantially coincident with said beam axis and being Ilocated within said magnetic held; la first hollow electrode surrounding la central region of said central electrode and electrically negative with .respect :to said central electrode; a third electrode comprising a lirst Aannular disc portion adjacent to one end of said central electrode, a second annular disc portion adjacent to the lother `end of said central electrode, both of said Iannular `disc portions being axially spaced from and insulated from said rst hollow electrode, and la tubular portion of said third electrode joining the periphenies tof said rst and second disc portions, said tubular portion encircling and being annularly spaced from said rst hollow electrode; a conductive member; a pin extending Ifrom said central electrode into sai-d conductive member to support said central electrode; an annular groove in the surface of said conductive member facing said central electrode; and a plurality Vof `openings extending from the bottom of said groove through said conductive member to permit electrons of said beam to pass therethrough.

13. rThe electron lens of claim l2 in which said conductive member comprises a `support for said third electrode.

14. rlhe electron lens of claim l2 in which said openings iare circular in cross-section :and are spaced apart in a circle which is coaxial with said axis.

l5. The electron lens of claim l2 in which said openings ia-re wedge-shaped and are separated by radial wall sections formed out of `said conductive member.

References Cited in the file of this patent UNITED STATES PATENTS 

3. IN AN ELECTRON BEAM DEVICE IN WHICH THE ELECTRON BEAM TRAVELS ALONG A BEAM AXIS, AN ELECTRON LENS ON SAID AXIS COMPRISING: MEANS FOR FORMING A SUBSTANTIALLY ROTATIONALLY SYMMETRIC MAGNETIC FIELD ON SAID AXIS; A CYLINDRICAL CENTRAL ELECTRODE WITHIN SAID FIELD AND COAXIAL WITH SAID BEAM AXIS; A FIRST ANNULAR ELECTRODE CONCENTRICALLY SPACED FROM SAID CENTRAL ELECTRODE AND COAXIAL THEREWITH AND NEGATIVELY ELECTRICALLY POLARIZED WITH RESPECT TO SAID CENTRAL ELECTRODE; AND A SECOND ANNULAR ELECTRODE COMPRISING A FIRST ANNULAR PORTION AXIALLY SPACED FROM SAID FIRST ANNULAR ELECTRODE IN ONE DIRECTION ALONG SAID BEAM AXIS AND A SECOND ANNULAR PORTION AXIALLY SPACED FROM SAID FIRST ANNULAR ELECTRODE IN THE OPPOSITE DIRECTION ALONG SAID BEAM AXIS, SAID SECOND ANNULAR ELECTRODE HAVING SUBSTANTIALLY THE SAME ELECTRICAL POTENTIAL AS SAID CENTRAL ELECTRODE AND BEING COAXIAL THEREWITH. 